THE DEVELOPMENT OF A PHAEOCYSTIS BLOOM IN A MESOCOSM EXPERIMENT IN RELATION TO NUTRIENTS, IRRADIANCE AND COEXISTING ALGAE

A widespread hypothesis is that man-induced eutrophication, an increase in nitrogen and/or phosphorus rather than silicon, gives a competitive advantage to the non-silicon using flagellates against the silicon dependent diatoms. Some authors suggest that such a mechanism could explain the intensification of the Phaeocystis blooms observed in the last decades in the coastal zones of the North Sea. A mesocosm experiment aiming to investigate this mechanism was performed in two 3 m(3) mesocosms. Light climate, mixing regime and nutrient concentrations reproduced conditions in Dutch coastal waters. Silicon was added to ensure the development of a phytoplankton development without Si limitation. Massive blooms of the colony forming Prymnesiophycean Phaeocystis sp. developed in both mesocosms and outcompeted the diatom community. Nutrients were continuously added and no nutrient limitation occurred during the bloom development. After four weeks, the Phaeocystis bloom collapsed and diatoms reappeared in both tanks. The dominance of Phaeocystis in a non silicon-limited environment constitutes an original observation which is in contradiction with the conclusions of some previously published works.